The present invention is generally related to the field of mass media information storage devices, and more particularly to a drive circuit and method for using a piezo actuator in both a charge mode and a voltage mode.
Hard disk drives are mass storage devices that include a magnetic storage media, e.g. rotating disks or platters, a spindle motor, read/write heads, an actuator, a pre-amplifier, a read channel, a write channel, a servo circuit, and control circuitry to control the operation of hard disk drive and to properly interface the hard disk drive to a host system or bus. FIG. 1 shows an example of a prior art disk drive mass storage system 10. Disk drive system 10 interfaces with and exchanges data with a host 32 during read and write operations. Disk drive system 10 includes a number of rotating platters 12 mounted on a base 14. The platters 12 are used to store data that is represented as magnetic transitions on the magnetic platters, with each platter 12 coupleable to a head 16 which transfers data to and from a preamplifier 26. The preamp 26 is coupled to a synchronously sampled data (SSD) channel 28 comprising a read channel and a write channel, and a control circuit 30. SSD channel 28 and control circuit 30 are used to process data being read from and written to platters 12, and to control the various operations of disk drive mass storage system 10. Host 32 exchanges digital data with control circuit 30.
Data is stored and retrieved from each side of the magnetic platters 12 by heads 16 which comprise a read head 18 and a write head 20 at the tip thereof. The conventional readhead 18 and writehead 20 comprise magneto-resistive heads adapted to read or write data from/to platters 12 when current is passed through them. Heads 16 are coupled to preamplifier 26 that serves as an interface between read/write heads 18/20 of disk/head assembly 10 and SSD channel 28. The preamp 26 provides amplification to the waveform data signals as needed. A preamp 26 may comprise a single chip containing a reader amplifier 27, a writer amplifier, fault detection circuitry, and a serial port, for example. Alternatively, the preamp 26 may comprise separate components rather than residing on a single chip.
Piezo actuators have improved performance when driven by quantities of charge versus the amount of voltage applied to it. The charge mode drive improves two important areas of performance, both well documented in the literature, namely, effects over temperature, and effects due to hysteresis. To operate a piezo actuator in a charge mode configuration, the drive circuit output must be placed in a high impedance, open loop state. Disadvantageously, once in an high impedance state, the piezo actuator can drift through charge loss, wander due to transducer effects, or simply wander due to a variety of effects and lack of feedback.
There is desired an improved piezo actuator drive circuit that can be advantageously operated in a charge mode configuration without the conventional drawbacks discussed above. Moreover, there is desired a piezo actuator drive allowing for a varying number of piezo elements, such as up to 8 elements.
The present invention achieves technical advantages as a piezo actuator driver having a low frequency compensation loop formed from the piezo actuator output to the driver input. The driver circuit is adapted to drive multiple piezo actuators, which number may vary from drive to drive. This circuit advantageously results in a closed loop system having a charge mode operation within a bandpass that can be tuned for desired operation. Below the bandpass turn on the circuit operates in the voltage mode, and the closed loop system restores the piezo output to a defined DC voltage and compensates for any wandering effects. The closed loop system also compensates for any DC current mismatches in the closed loop configuration such that the piezo output is centered around a desired DC operating point.
The present invention advantageously uses a second amplifier with a resistor/capacitor configured as an integrator to set up the DC restore for the piezo driver and produce a highpass response. At high frequencies above the cut on of the loop, the driver is advantageously in a charge mode drive. However, below the cut on frequency, the driver is in a voltage mode and restoring the output to a commanded DC voltage. This circuit is compatible with a voltage mode drive, yet provides a charged mode solution without the conventional drawbacks including wandering output DC voltage. Offsets are compensated by the loop.